About two inches taller than the high elf average. I did research and math, feel free to refute it, but, Imperials most resemble the real world race of Germans. Last checked in 2009 the average german was about 5'10 or 180 cm. Altmer, in all the games that have height modifiers, in order are 1.1 1.1 and 1.08 times the height of an imperial. That comes out to 1.0933333etc on average, and for simplicity's sake (Because I like science, but approximate height of humanoids is not an exact science), I rounded to 1.1 flat and that puts Altmer at an average of 198 cm or approximately 6 and a half feet. I don't know if you have as big a hard on for math as I do, but good job, a 2 inch deviation from the average at that height is almost negligible.
Someone has corrected me, I was wrong and I apologize. The Creation Kit makers actually show that a 1.0 character is approximately 128 units tall, or about 6 feet. So characters are in fact taller than I calculated.